The present invention relates to computer software utility programs, and more specifically to programs in the field of computer aided design (CAD), computer aided manufacturing (CAM), and computer aided engineering (CAE), and in particular the use of such systems for numerically controlled machining and the generation of a toolpath for such numerically controlled machining. The invention is particularly useful with, although not restricted to, high speed machines for High Speed Milling (HSM).
The general objective to be achieved in defining a toolpath is to ensure that all the material that has to be cut from a workpiece to achieve a pocket of a desired shape is indeed cut when the tool has followed the defined path. A toolpath generator generates planar tool motion in a succession of one or more parallel planes going from the top of the pocket down to the bottom.
Several types of toolpaths have been proposed in the art. One is called the helical toolpath which is comprised of a series of passes where each pass is derived from the previous one by applying a predefined offset. This is depicted in FIG. 1 in the left-hand view, entitled xe2x80x9cClassical Helical Toolpath.xe2x80x9d The passes can start from the external side of the pocket (inward offsetting) or from the center of the pocket (outward offsetting). The toolpath obtained with classical toolpath generators give sharp angles and big variations in cutting directions. New machines dedicated to HSM are now able to reach high spindle speeds with fast cutting speed. For such machines, sharp angles and big variations in cutting directions should be avoided to prevent tool or machine breakage.
An important parameter affecting efficiency when cutting a pocket in a part is the distance between successive passes expressed in terms of its relation to the diameter of the tool. Obviously, the distance cannot be greater than the tool diameter as this would necessarily leave some material uncut. On the other hand, the efficiency of the machine is severely reduced if the distance is much smaller than the diameter. Most toolpath generators currently on the market provide for a distance between adjacent passes which is half the diameter of the tool. It would be advantageous to increase this distance to have it equal to or as close as possible to the diameter of the tool.
Another difficulty with the currently known helical toolpath is the presence of sharp angles and big variations in the direction of cutting. This is not desirable even with conventional milling machines, and becomes a very serious problem with new high speed milling machines since it can lead to tool or machine breakage. To avoid this, current toolpath generators provide for a lower rotation speed of the tool when approaching a change of direction. However, this leads to a considerable loss of time in the process.
A further difficulty in HSM is the occurrence of segments in the toolpath where the tool passes a location that has already been cut. This causes shock loading of the machine as the tool suddenly finds itself in an area from which the material has already been removed. In known toolpath generators, this can occur, for example, when the tool has finished one pass and moves to the next one.
Although the classical helical toolpath works reasonably well with simple pockets, such as the parallelogram of FIG. 1, it is far less efficient when the desired pocket exhibits a complex shape such as the shape of the pocket of FIG. 2.
There is therefore a need for a toolpath generator which addresses the above problems, and provides an efficient toolpath which protects the cutting machine and tool, while at the same time minimizing cutting time.
Accordingly, the present invention provides a system, method and apparatus for determining an optimized toolpath in a milling situation. According to the invention, the machining time for a given task is minimized, wear of both the machine and the cutting tool is reduced and shock loading is avoided. An improved toolpath is generated which ensures a progressive tool engagement and regular distribution of passes, taking into account a minimum circle radius, with no hard direction variation, leading to regular cutting force variation. The toolpath generator has ramping approach material capabilities and is able to keep contact with the material throughout the cutting. In addition, a large portion of the tool diameter is engaged in material during the cutting.
The new helical toolpath generator of the present invention machines the material in several horizontal slices. All the cutting motions in a same plane are based on rules so that:
Tool engagement in material is progressive for each approach
The toolpath respects a minimum radius constraint
During cutting motion, the tool stays in contact with material
The tool retracts from the part along a circular-based helix.
A toolpath generated according to the present invention is therefore useful for controlling High Speed Milling machines, but can also be used for usual machines. Using this technology results in a dramatic decrease in machining time and reduces the wear on the machine and the cutting tool. An important feature of the present invention is the concept of focal areas, for determining the relative centroid of portions of the pocket to be cut, and the concept of machining beds for defining portions of the pocket to be cut.
A depiction of a toolpath generated according to the present invention is shown in FIG. 1, in the right-hand view, entitled, xe2x80x9cNew Helical Toolpath for HSM.xe2x80x9d